1. Field of the Invention
The present invention relates generally to two-dimensional CCD (charge-coupled device) imaging device, and more specifically to such a CCD device capable of efficiently discharging undesired charge packets. The present invention is particularly suitable for CCD image sensors where large amounts of non-usable charge packets must be quickly discharged.
2. Description of the Related Art
Japanese Laid-Open Patent Specification Sho 62-154881 discloses a two-dimensional charge transfer device comprising a plurality of vertical registers for receiving electrons, or "charge packets" from a two-dimensional array of photodiodes and shifting them down to a horizontal register. The horizontal register includes overflow barrier regions and an overflow drain region. When excessive charge packets are produced, they overflow the barrier regions and are discharged through the drain region to the outside, while useful charge packets are diverted to a utilization circuit. The overflow barrier regions of this prior art device are formed into the shape of narrow channels to take advantage of what is known as the narrow channel effect of a semiconductor junction on a threshold voltage, i.e., when the channel width is reduced, the threshold voltage of the junction increases. Due to the narrow channel effect, the overflow channel regions are at such a potential that only surplus electrons are discharged to the outside.
However, the narrow channel structure produces a throttling effect on charge packets. Therefore, the speed at which undesirable excess charge packets are discharged is not satisfactory for most applications.
Japanese Laid-Open Patent Specification Sho 63-254767 discloses a single-layer diffused overflow barrier region extending between the overflow drain region and the edges of horizontal-register channels remote from the vertical registers. However, the disadvantage of the prior art is that the forming of this barrier region requires an additional photoresist process.